1. Field of the Invention
The present invention relates to a method of driving a DC/DC converter for increasing and/or reducing a DC voltage, a method of controlling a DC/DC converter apparatus including a DC/DC converter, a method of controlling a driving operation of a vehicle incorporating a DC/DC converter apparatus therein, and a method of controlling a fuel cell system incorporating a DC/DC converter apparatus.
2. Description of the Related Art
There have heretofore been widely used DC/DC converter devices having switching devices therein such as MOSFETs, IGBTs, or the like.
For example, there has been proposed a vehicle (hereinafter referred to as an “electric vehicle”) incorporating a DC/DC converter apparatus for increasing and reducing a DC voltage, which is connected between an electricity storage device and a motor that is energized by an inverter. On the electric vehicle, when the motor is energized, the voltage across the electricity storage device is increased by the DC/DC converter apparatus and applied to the inverter. When the motor regenerates electric power, the regenerated voltage from the inverter is lowered by the DC/DC converter apparatus and applied to charge the electricity storage device.
There has also been proposed a vehicle (hereinafter referred to as a “fuel cell vehicle”), which also uses a motor as a propulsive source. The fuel cell vehicle includes a fuel cell directly connected to the motor, which is energized by an inverter. A DC/DC converter apparatus for increasing and reducing a DC voltage is connected between an electricity storage device and the junction between the fuel cell and the motor. The fuel cell is used as a main power supply, while the electricity storage device is used as an auxiliary power supply for assisting the main power supply.
On the fuel cell vehicle, when the motor is energized, the voltage across the fuel cell and the voltage across the electricity storage device, which has been increased by the DC/DC converter apparatus, are added together, and the summed voltage is applied to the inverter. When the motor regenerates electric power, the regenerated voltage from the inverter is lowered by the DC/DC converter apparatus and applied to charge the electricity storage device. If the electric power generated by the fuel cell contains an excessive amount of electric power, then the electric power is lowered in voltage and is applied to charge the electricity storage device.
WO 02/093730 discloses a charging and discharging control apparatus having an AC power supply, a diode bridge connected to the AC power supply, an inverter connected to the diode bridge, an electric motor energizable by the inverter, a voltage increasing and reducing circuit connected to the inverter, and a secondary battery connected to the voltage increasing and reducing circuit. The disclosed charging and discharging control apparatus operates such that, when a current flowing from the AC power supply through the diode bridge into the inverter is cut off, the secondary battery supplies the electric motor with all the electric power required to energize the electric motor through the voltage increasing and reducing circuit and the inverter, while the electric motor is in a propulsive power mode. Further, the secondary battery is charged with all the regenerated electric power while the electric motor is in a regenerative mode. In the voltage increasing and reducing circuit, voltage increasing switching devices and voltage reducing switching devices are alternately turned on and off, with dead times being interposed therebetween, regardless of the propulsive power mode and the regenerative mode of the electric motor. In this manner, the current flowing through the reactor is made continuously active, making it unnecessary to detect interruptions of the current or to determine the propulsive power mode and the regenerative mode.
In the DC/DC converter including the voltage increasing and reducing circuit, a drive signal having a predetermined duty ratio is supplied to the switching devices in order to energize the switching devices for increasing and/or reducing the voltage. The duty ratio is PWM-controlled so that the output voltage of the DC/DC converter will become a target voltage, i.e., a target value for the output voltage.
However, when a duty ratio depending on the target voltage (hereinafter referred to as a “target duty ratio”) is changed to adjust the output voltage, the output voltage may change non-linearly depending on the change in the duty ratio, and the DC/DC converter may produce an output voltage which deviates from the voltage corresponding to the duty ratio. WO 02/093730 indicates nothing concerning prevention of variations in the output voltage.
Japanese Laid-Open Patent Publication No. 06-311736 discloses a DC/DC converter, wherein when the DC/DC converter produces a low output voltage at the time the voltage is to be reduced, if the pulse duration of output pulses of a pulsed drive signal output from a control means for energizing switching devices is shorter than an ON duration, which has been set by a minimum ON duration setting means, then the switching devices are energized by a pulsed drive signal having the preset ON duration.
According to Japanese Laid-Open Patent Publication No. 06-311736, the pulsed drive signal for energizing the switching devices is controlled such that the ON time of the pulsed drive signal will not be shorter than the minimum ON time of the switching devices, i.e., an ON time threshold value for reliably turning on the switching devices (changing the switching devices from a turned-off state to a turned-on state). However, there is nothing proposed in Japanese Laid-Open Patent Publication No. 06-311736 that deals with energizing the switching devices within a time that is shorter than the minimum ON time.
When the DC/DC converter is connected between the electricity storage device and the fuel cell, the duty ratio of the drive signal in a nearly directly coupled state, in which the voltage of the DC/DC converter across its terminals connected to the electricity storage device and the voltage of the DC/DC converter across its terminals connected to the fuel cell are substantially equal to each other, comprises a duty ratio that depends on an ON time shorter than the minimum ON time. When the target duty ratio is changed to adjust the output voltage, the duty ratio depending on the ON time, which is shorter than the minimum ON time, is unable to reliably turn on the switching devices. As a result, the output voltage does not change continuously (linearly) when the duty ratio changes, and the DC/DC converter produces an output voltage that deviates from the voltage corresponding to the duty ratio.
For reliably turning on the switching devices in an ON time that is shorter than the minimum ON time, to thereby prevent the DC/DC converter from producing an output voltage that deviates from the voltage corresponding to the duty ratio, it may be proposed (1) to lower the switching frequency (to increase the switching period) of the switching devices so as to reduce the proportion of the ON time and the duty ratio, or (2) to secure the ON time and reduce the duty ratio under a PFM control.
According to proposal (1), however, when the switching frequency is lowered to within an audible frequency range, sounds produced by the switching operations of the switching devices are heard as offensive noises by occupants of the electric vehicle or fuel cell vehicle, which incorporates the DC/DC converter therein. Therefore, the commercial value of the DC/DC converter, the DC/DC converter apparatus, the electric vehicle, and the fuel cell vehicle tends to be lowered. Furthermore, since the switching frequency is lowered, the current flowing through the reactor tends to contain larger ripples. If the reactor is increased in size to minimize ripples, then the DC/DC converter including the reactor is liable to become larger in size and weight, as well as cost.
According to proposal (2), if the frequency of the drive signal is increased under a PFM control, then the switching devices suffer from an increased switching loss. Also, if the frequency of the drive signal is reduced under the PFM control, then the same problems that occur with proposal (1) tend to arise.
Consequently, proposals (1) and (2) are unable to reliably turn on the switching devices in an ON time that is shorter than the minimum ON time.